Sheet-metal pieces are here generally defined as pieces to be shaped, in particular stamped, bent and/or embossed pieces of sheet metal, metal strips or the like. The drawn areas comprise areas on such shaped pieces which are deformed to a greater extent. The drawn areas are similar to a sleeve or bush and are often called collars. As a rule, they are provided with an inner thread and are then used as substitutes for nuts. Such shaped pieces are used in various areas of metal construction, namely in the manufacture of vehicles and apparatus, but also in the electrical industry and other branches of industry.
The shaping operation is in the form of a deep drawing operation. Accordingly, as a rule the shaping tools comprise a lower part, which is a so-called drawing die with a suitable recess and an ejector contained therein and an as upper part, which is a so-called hold-down clamp or blank holder with a drawing punch contained therein.
To produce the desired drawn bores, the sheet-metal pieces are increasingly deformed in steps, namely starting with a flat shape, i.e. a flange, a round or a blank. The shaping operation of interest here may itself be a part of a total manufacturing process, namely the fabrication of finished shaped pieces from semi-finished products. All in all, various working steps may be used, such as stamping, bending, drawing, punching, embossing, cutting. As already mentioned, the entire shaping takes place step by step, i.e. a number of shaping tools are required to attain increasingly greater degrees of deformation. These tools must be passed successively in contact with the workpiece. Regarding the details of the shaping process, the piece to be shaped first must be placed o the lower part of the respective shaping tool and positioned there. As a rule, the lower part has a special receptacle for the workpiece. Underneath the receptacle is a recess which allows a bulging of the sheet-metal workpiece in accordance with the corresponding degree of deformation. In prior art methods, before being deformed at all, the portion of the sheet-metal piece which directly borders the area to be crimped, i.e., drawn, is pressed flat against the lower part with a hold-down clamp or blank holder being placed firmly on the sheet-metal piece and a certain surface pressure being generated afterwards. To reduce shock, the pressing forces are in general generated by springs.
After the sheet-metal piece has been fixed on the lower part, the respectively intended deformation is started by lowering the drawing punch out of the upper part in the direction of the lower part. The desired shaping step with a view to a finished shape is attained in cooperation with the drawing die. After the respective shaping, the sheet-metal piece can be ejected from the drawing die by means of the ejector.
The sequence for preparing, initiating and operating the desired deformation has been always the same in accordance with the custom so far and has been considered mandatory. In the same way, the amount of deformation considered from station to station is subject to fixed rules. For this purpose, the diameter of the round and the recess or drawing punch and the depth of the punch penetration have been brought into a fixed mathematical relationship to each other.
By means of the known process it is possible to provide threaded drawn portions in excess of the normal height of nuts (for example, 6 mm for M 6 threads). However, this must be qualified by stating that this is only successful in connection with rounds or flanges of small area. In part, the wall thickness of the drawn area is reduced in comparison to the thickness of the original material. This is because the pressing flat of the portions of the sheet metal piece bordering the area to be drawn prevents the flow of metal from this bordering portion into the drawn portion. Therefore, since the amount of metal available for drawing is limited by the pressing flat of the sheet metal piece, the greater the height of the drawing, the thinner the wall thickness must be. Quite often losses in wall thickness up to 25%, occasionally even up to 50%, based on the original material, must be tolerated. However, this may be satisfactory in drawn bores for normal screw connections.
But with increased collar height, the wall thickness if reduced to the extent that resistance to tear-out falls below the permissible minimum. The customary method thus permits normal screw connections at best, but not screw connections requiring very high tear-out resistance. This is true to even a greater extent for sheet-metal parts (rounds) having larger areas. It is not even possible to manufacture drawn bores for normal stress. In such cases, weld nuts or threaded sheet-metal nuts will have to be welded on instead, which, of course, requires extensive effort and increased cost.